We shall investigate the role of myoglobin in the oxygen economy of isolated red skeletal and cardiac muscle fibers. Specifically: What fraction of the oxygen uptake of cardiac and red skeletal muscle fibers is dependent on myoglobin-mediated transport at physiological oxygen pressure? How effectively does myoglobin-delivered oxygen support ATP generation? What are the conditions of oxygen pressure, myoglobin oxygenation and oxygen flux required for optimal mitochondrial function. And, does carbon monoxide, at concentrations encountered in urban atmospheres, impair myoglobin-mediated oxygen delivery. We will measure the oxygen uptake of isolated cells from skeletal and cardiac muscle in families of steady states of constant oxygen pressure. The extent of myoglobin oxygenation will be measured spectrophotometrically. In order to overcome limitations of oxygen delivery to separated muscle cells we will add oxygen-binding proteins to the suspending medium. These will also serve as internal indicators of oxygen pressure. Oxygen consumption will be followed by simultaneous polarographic measurement of oxygen pressure and spectrophotometric measurement of fractional oxygenation of the oxygen-binding protein in the medium. We will abolish the oxygen-binding functions of intracellular myoglobin by specific chemical treatment, and determine the quantitative contribution of functional myoglobin toward the oxygen uptake and energy production of red skeletal and cardiac muscle cells.